1. Field of the Invention
The present invention relates to a light source for optical transmission systems and optical networks based on the wavelength-division multiplexing (WDM) technology, and more particularly to a light source employing a Fabry-Perot laser diode (F-P LD) whose output wavelength is locked by an externally injected incoherent light.
The present invention also relates to WDM transmission systems and WDM passive optical networks using the above-described light sources.
2. Description of the Related Art
Recently, WDM transmission systems are widely deployed to meet the ever-growing bandwidth demands incurred by the explosion of the data traffic. In particular, WDM transmission systems begin to be deployed in metropolitan networks and access networks to accommodate wide-band services such as an electronic commerce, a cable TV, a video conference, and son on.
FIG. 1 shows a configuration of conventional WDM transmission system.
The source node is equipped with multiple transmitters (TXs) with different wavelengths (λ1˜λN) and a N×1 multiplexer (MUX) and the destination node is equipped with an 1×N demultiplexer (DMUX) and multiple receivers (RXs). The source node and the destination node are connected through an single strand of optical link composed of optical fibers and optical amplifiers.
In WDM transmission systems described above, communication channels between the source node and the destination node are distinguished one another by their wavelengths. Thus, a unique wavelength is allocated each transmitter-receiver pair. The light source of transmitter must have the unique wavelength with long-term stability and a large side mode suppression ratio (SMSR) to minimize the interference between neighboring channels. In addition, it is desirable that the light source provides a sufficient output power and has a narrow spectral width.
A representative light source which satisfies the requirements mentioned above is a distributed feedback laser diode (DFB LD). However, since a distributed feedback laser diode is expensive, incoherent light sources are usually used in an access network in which the main concern is the economical competitiveness
The incoherent light sources, such as a light emitting diode (LED), a super-luminescent diode (SLD), and an optical fiber amplifier generating amplified spontaneous emission (ASE), have been used in WDM transmission systems through a spectrum-slicing application. The LED can be fabricated at low cost and modulated directly. However, the output power of LED is not sufficient to accommodate many channels through a spectrum-slicing application. The SLD is costly although it can provide much higher output power than the LED. The optical fiber amplifier can provide a strong incoherent light, ASE, but it requires expensive external modulators.
The F-P LD can provide much higher output power than the LED at the comparable cost with the LED. However, its output is multi-mode and the output power of each mode fluctuates randomly with the time due to the mode hopping and the mode partitioning. Therefore, it has been used in optical transmission systems based on time-division multiplexing technology (TDM) rather than WDM technology. Its application wavelength region was also limited near the zero dispersion wavelength of the optical fiber.